The document discusses different disk scheduling algorithms used in operating systems to efficiently manage requests to read and write data from disk drives. It describes seek time and rotational latency as the components of access time. It then explains first come first serve (FCFS), shortest seek time first (SSTF), SCAN, C-SCAN, LOOK, and C-LOOK algorithms. FCFS handles requests sequentially, SSTF selects the nearest request to reduce head movement, while SCAN and C-SCAN algorithms move the disk head across cylinders in single direction and wrap around. LOOK and C-LOOK reverse direction only after reaching the last request.

1. Operating System ALA
Semester - 4
CSE (A)

3. The operating system is responsible for using hardware
efficiently.
For the disk drives, this means having a fast access time
and disk bandwidth.
Access time has two major components:-
Seek time is the time for the disk are to move the
heads to the cylinder containing the desired sector.
Rotational latency is the additional time waiting for
the disk to rotate the desired sector to the disk head.

4. Storage-area Network:-

5. Disk bandwidth is the total number of bytes transferred,
divided by the total time between the first request for
service and the completion of the last transfer.
Whenever a process needs I/O to or from the disk, it
issues a system call to the OS. The request specifies
several pieces of information:
->Whether this operation is input or output
->What the disk address for the transfer is
->What the memory address for the transfer is
->What the number of sectors to be
transferred is

6. Given a request queue of block numbers, minimize summed
seek distance
Block numbers are translated into track and sector
numbers
Higher block numbers correspond to higher track
numbers
Sector numbers may not be known exactly, but can be
computed for analysis
Example, for a disk with range 0-199:
98, 183, 37, 122, 14, 124, 65, 67
Head pointer initially at 53

7. Illustration shows total head movement of 640 cylinders.

8. Handle I/O requests sequentially.
This algorithm is intrinsically fair, but does not provide
fastest service.
Approaches random scheduling in performance if there are
many processes/requests.
Suffers from global zigzag effect.

9. Illustration shows total head movement of 236 cylinders.

10. Selects the request with the minimum seek time from
the current head position.
In other words, SSTF chooses the pending request
closest to the current head position.
It’s based in favor of the middle cylinders requests.
SSTF scheduling is a form of SJF scheduling; may cause
starvation of some requests.

11. Illustration shows total head movement of 208 cylinders.

12. The disk arm starts at one end of the disk, and moves
toward the other end, servicing requests until it gets to the
other end of the disk, where the head movement is
reversed and servicing continues.
It moves in both directions until both ends.
Tends to stay more at the ends so more fair to the
extreme cylinder requests.

14. The head moves from one end of the disk to the
other, servicing requests as it goes. When it reaches the
other end, however, it immediately returns to the
beginning of the disk, without servicing any requests on
the return trip.
Treats the cylinders as a circular list that wraps around
from the last cylinder to the first one.
Provides a more uniform wait time than SCAN; it
treats all cylinders in the same manner.

15. The disk arm starts at the first I/O request on the disk,
and moves toward the last I/O request on the other
end, servicing requests until it gets to the other extreme
I/O request on the disk, where the head movement is
reversed and servicing continues.
It moves in both directions until both last I/O requests;
more inclined to serve the middle cylinder requests.

17. Look version of C-Scan.
Arm only goes as far as the last request in each
direction, then reverses direction immediately, without
first going all the way to the end of the disk.
In general, Circular versions are more fair but pay with a
larger total seek time.
Scan versions have a larger total seek time than the
corresponding Look versions.